At present, a large variety of methods of constructing composite piles to improve ground have been suggested and put to practical use. For example, there has been suggested a technology in which the ground and slurry are mechanically stirred and mixed by a stirring mixing device while injecting the slurry including cement as a main component into the ground, to construct a soil cement column body. Furthermore, a steel pipe pile with spiral blades is twisted and inserted into the soil cement column body prior to curing, and integrated with the column body, thereby constructing the composite pile (e.g., see Patent Documents 1 and 2).